1. Technical Field of the Invention
The present invention relates generally to multiple power converters used in conjunction, and relates more particularly to power converters used in conjunction with interleaved phases.
2. Description of Related Art
Performance improvements in interleaved, multiphase power supplies result from advantages such as reduced input current ripple, reduced peak output current and higher frequency output ripple current, the output ripple being thus easier to filter. A particular type of switching power supply has a variable switching frequency to obtain desired power supply output characteristics based on input power. Due to the variable frequency nature of the power supply switching, it can be challenging to synchronize the various phases to obtain one or more of the above-described advantages.
Obtaining high efficiency in switching power supplies is also challenging, and can be improved with multiple interleaved phases when properly synchronized. Another aspect for improving efficiency of switching power supplies is to achieve zero voltage switching and zero current switching, so that switching losses can be reduced, especially at high frequencies. Various types of switching modes are employed in switching power supplies to achieve desired output characteristics. For example, a switching power supply may operate in continuous, discontinuous or transition mode, depending on the desired operating characteristics for the power supply. One advantage to transition mode operation is the potential for zero voltage and/or zero current switching in the power supply.
Transition mode operation can also contribute to improving the power supply power factor by keeping the input voltage in phase with the input current. However, the variable frequency nature of the switching power supply makes synchronization of the various power phases somewhat challenging as frequency changes to deliver desired output power characteristics. It is desirable to properly align the phases of an interleaved power supply to obtain the advantages of a properly synchronized multiphase interleaved power supply. For example, if a multiphase interleaved power supply has two phases, the waveforms of the phases should be 180° apart. For a three-phase interleaved power supply, the waveforms of the phases should be maintained 120° apart. In general, the phase angle separation is equal to 360°/N, where N is the number of phases in the interleaved power supply.
One way to correct for mismatch in phase separation is to employ a Phase Locked Loop (PLL) to maintain an appropriate phase angle separation. Such a concept is illustrated in U.S. Pat. No. 5,793,191, where a slave stage of a power converter is maintained 180° out of phase with a master power converter stage. One drawback to this approach is the challenge of acquiring and maintaining a phase lock over a wide range of conditions. The PLL capture range must encompass the difference between the free-running frequencies of the master and slave(s), which may prove difficult or costly in practice.